1. Field of the Invention
The present invention refers to a method of bending a workpiece to a predetermined bending angle by means of a bending apparatus which comprises a movable bending bar and a fixed bottom die having a die bottom member, the height position thereof being adjustable in accordance with the bending angle to be achieved.
Particularly, the present invention refers to a method of bending a workpiece to a predetermined bending angle by means of a bending apparatus which comprises a movable bending bar resiliently supported on a pressurerized fluid cushion and a fixed bottom die having a die bottom member, the height position thereof being adjustable in accordance with the bending angle to be achieved.
If a piece of sheet metal has to be bent by means of a bending bar and a cooperating bending die, it is quite difficult to precisely realize a predetermined bending angle. Although the bending angle can be theoretically calculated if the width of the upper aperture of the bottom die and the depth of penetration of the bending bar into the upper aperture is known; in practice, however, the calculated results cannot be achieved because the actual bending angle shows greater or smaller deviations from the nominal value due to inaccuracies of the bending bar movement as well as due to variations of quality and thickness of the sheet material to be bent.
2. Prior Art
A certain improvement is possible by the use of a bending apparatus in which the bending angle can be adjusted and changing easily and exactly. Preferably, such an apparatus comprises a bottom die having an upper longitudinal aperture directed towards the bending bar and a die bottom member which can be adusted in height position. The bending angle is exactly defined by the height position of the die bottom member, and by re-adusting said position different bending angles can be realized without the need of exchanging the bottom die.
The bottom die comprises a longitudinally running groove whose opening is directed towards the bending bar, and the bending angle is defined by the distance between the two fixed upper edges of the groove running parallel to each other and by the height position of the top surface of the movable die bottom member. However, also in this case, a resilient back movement of the two legs of the bent sheet metal workpiece can be observed as soon as the bending force is released from the workpiece with the result that the actual bending angle does not coincide with the theoretically calculated value of the bending angle.
Thus, it is imperative to run test bending operations with a specified quality and thickness of sheet metal in order to determine the deviation of the actual bending angle from the nominal value. As soon as the deviation is available, the path of movement of the bending bar towards the bottom die and/or the height position of the die bottom member can be corrected in accordance with the deivation factor before the production of the bent sheet metal workpieces finally starts.